This invention relates to wide-angle microwave lens for line source radar antenna applications and in particular to such lens which permit a resulting radiated beam to be scanned in small spaced increments while the array factor remains essentially constant.
Wide-angle microwave lens used as an antenna line source have been known for a long time. A typical such lens is comprised of a pair of flat parallel conducting plates whch comprise an RF transmission line fed by an input means of electromagnetic energy located on a focal arc, a plurality of coaxial transmission lines connected to output probes which extract energy from the parallel plate region, and a linear array of radiating elements fed individually by the coaxial transmission lines and radiating energy into space. As the input means is moved along the focal arc a resulting beam which is radiated by the radiating elements is scanned through a field of view. A wide-angle microwave lens antenna of this type has been described in U.S. Pat. No. 3,170,158 for "Multiple Beam Radar System" by Walter Rotman and in the technical paper "Wide Angle Microwave Lens for Line Source Applications" by Rotman and Turner on pages 623 to 632 of the November 1963 IEEE Transactions on Antennas and Propagation, and has come to be known as a Rotman lens antenna. It has been proposed to use a Rotman lens antenna in a microwave landing system (MLS) where the antenna is used to sweep a radiated beam through space at a known rate through known bounds. Thus, an aircraft periodically illuminated by the radiated beam could determine from the characteristics of the illumination its position in space with respect to the radiating antenna. If the radiated beam is swept horizontally then the aircraft could determine its azimuth with respect to the radiating antenna, while a beam swept vertically would provide elevation information to the aircraft, as known to those skilled in the art. Usually one antenna is arranged to sweep a beam horizontally and the other antenna is arranged to sweep a beam vertically, thus providing for practical purposes simultaneous azimuth and elevation information at an illuminated aircraft.
If the input means comprises a plurality of feed probes arranged along the focal arc and the various feed probes are energized so as to feed electromagnetic energy into the parallel plate region one at a time consecutively, the resulting beam will scan through space in distinct steps whose angular separation is directly related to the angular separation between adjacent feed probes. It is desirable, of course, that the aforementioned steps be as small as possible since positional uncertainty at the illuminated aircraft increases as the angular separation between consecutive beams, and hence distance between adjacent feed probes, increases. In short, a smoothly commutated beam provides the best degree of positional certainty at the illuminated aircraft, thus dictating relatively close feed probe spacing. However, if the feed probes are positioned too close to one another adjacent probes will be parasitic to an energized probe, thus distorting its resulting beam shape. One means of providing a well shaped smoothly commutating beam is through the use of a single feed probe instead of the above described plurality and physically moving the single probe along the focal arc of the lens. This type of scanning probe, however, introduces an undesirable mechanism to produce the mechanical motion.
In the aforementioned related patent application it was explained that a well shaped beam can be scanned through space in small steps by providing a plurality of feed probes along the focal arc of the antenna spaced so that the beam resulting from exciting any feed probe is orthogonal to the beam resulting from exciting an adjacent feed probe. A beam is then actually scanned through space by providing input power to the lens antenna through an adjacent number of feed probes simultaneously in accordance with a predetermined weighting schedule. As the weights are varied the beam scans through space.
It has been found that when using the invention of the aforementioned related patent application, it is possible to further improve the shape of the resulting scanned radiated beam by optimizing the phase of the energy supplied by the feed probes to the parallel plate region of the microwave lens antenna.